A power conversion device which outputs a large amount of power converts high voltage. Therefore, it is necessary to increase the withstand voltage of the power conversion device by using a switching element that has a high withstand voltage, or by connecting in series switching elements that do not have a high withstand voltage. Furthermore, it is necessary to increase the output voltage of the power conversion device by providing the power conversion device in multistages using a transformer.
In the case where the withstand voltage of the switching device is high, the switching loss of the switching device is large. Therefore, in some cases, by switching the switching element only once per cycle of an output frequency (one pulse) and shifting phases, a one-pulse control for eliminating a specific harmonic is carried out. This one-pulse control has an advantage in that the loss caused by switching the switching element can be reduced, and the harmonic can be reduced as well.
When applying Pulse Width Modulation (PWM) control to a Neutral-Point-Clamped (NPC) power conversion device, a harmonic component can be significantly reduced since the output voltage becomes even closer to the sine wave. However, in the case of using this power conversion device with a large amount of power, the loss caused by switching the switching element becomes greater.